The present invention relates to a resistance unit for motor speed control, and more particularly to a resistance unit having a temperature fuse between a resistance and a terminal for breaking a resistance circuit when the motor reaches the permissible maximum temperature.
In a conventional motor speed control system, there is no means for protecting a motor from overheating due to overload, mechanical defects and so on. Accordingly, not only may the motor be damaged, but also the instruments associated therewith.
A prior art resistance circuit having a temperature fuse is set forth in Japanese patent unexamined publication no. 2-304829 in which the temperature fuse is electrically connected to an open portion of a resistance. However, this type resistance circuit has a drawback in that the temperature fuse is not rapidly activated under the predetermined melting temperature, and has a further disadvantage in that it sometimes can not completely deactivate the circuit.